Separation of cerium from other rare earths



United States Patent 3,351,424 SEPARATION OF CERIUM FROM OTHER RARE EARTHS Lane A. Bray and Francis P. Roberts, Richland, Wash.,

assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Apr. 20, 1964, Ser. No. 361,629

5 Claims. (CI. 23-22) ABSTRACT OF THE DISCLOSURE Cerium is separated from a radioactive aqueous solution containing cerium and lanthanide rare earths by coextracting cerium and lanthanide rare earth values into a dialkyl phosphoric acid, separating the organic from the aqueous phase and stripping the lanthanide rare earths, except for cerium, from the organic phase by contacting the organic phase with an aqueous solution 1 to 2 molar in mineral acid, 0.01 to 0.02 molar in silver ion and about 0.2 molar in persulfate ion.

This invention deals with an improved process of separating radioactive isotopes of cerium from those of other lanthanide rare earths by first coextracting the cerium and the other rare earths into a water-immiscible organic solvent; contacting the organic solution obtained with an aqueous solution of alkali persulfate plus silver catalyst, whereby cerium is oxidized to the tetravalent state and held as such in the organic solution, While the other, trivalent, rare earths are back-extracted into the aqueous persulfate-silver solution; separating the organic solution from the aqueous solution; allowing the tetravalent cerium in the organic solution to reconvert to the trivalent state; and back-extracting the trivalent cerium into a dilute aqueous mineral acid solution.

Solutions containing radioactive cerium and other lanthanide rare earths occur in waste products, such as aqueous or organic solutions or precipitates, obtained in the processing of neutron-irradiated uranium. Before further processing such waste products for the separation of the various radioactive fission products, they are usually stored for some time to allow short-lived radioactive, and therefore hazardous, isotopes, for instance Pm to decay; the waste products then can be handled more easily and can be processed with ion exchange resins with reduced or no damage to the latter by radioactivity. Removal of some radioactive isotopes by processing or decay also allows the storage of a greater quantity of promethium and a reduction of the shielding equipment. Pure cerium as it is obtained by the process of this invention has utility for electrical sources for satellites. Ce which is one of the radioactive isotopes usually present in the waste products described above, has too long a half-life, however, to make its decay by storage possible within a reasonable period of time; it therefore must be removed by chemical processing.

It has been tried heretofore to separate cerium by oxidation to its tetravalent state and solvent extraction of the trivalent rare earths away from the tetravalent cerium. These processes performed satisfactorily in non-radio active solutions; but they were entirely unsuccessful for radioactive solutions. Most likely the radioactivity, in particular the gamma rays, destroyed the oxidizing agent and it possibly also decomposed the organic solvent used as the extractant. When, for instance, potassium permanganate was the oxidizing agent, it had to be replenished in the radioactive solution almost continuously. Similar experiences were had with lead dioxide and also with sodium bismuthate as the oxidizing agent for extraction, and also 3,351,424 Patented Nov. 7, 1967 use becomes impracticable. Higher temperatures are or.

desirable, because then the decomposing effect of the radioactivity becomes too great. When silver ion catalyst Was added to the persulfate, the oxidation rate was greater; however, the tetravalent cerium was not very stable and was easily reduced to the trivalent state so that its extraction into and/or retention in the organic solvent was not quantitative and a poor separation from the trivalent rare earths resulted.

It is an object of this invention to provide a process of separating cerium from other lanthanide rare earths present together in radioactive aqueous solutions by solvent extraction in which the cerium can be maintained in the tetravalent state almost quantitatively and for a relatively long period of time, so that the trivalent rare earths can be readily and selectively extracted away from it by a long and thorough contact with the extractant.

It is another object of this invention to provide a process of separating cerium from other lanthanide rare earths present together in radioactive aqueous solutions by solvent extraction in which the oxidation and extraction can be carried out at room temperature.

It is finally an object of this invention to provide a process of separating cerium from other lanthanide rare earths present together in radioactive aqueous solutions by solvent extraction in which the organic solvent and the oxidizing agent withstand the decomposing effect of the radioactivity present.

It was found against all expectations that, if certain critical concentrations of the silver catalyst, the persulfate anion and the acidity are observed, a quantitative conversion and retention of cerium to the tetravalent state can be obtained for a considerable period of time, and in particular sufficiently long to carry out the extraction of the trivalent lanthanide rare earths away from the tetravalent cerium leisurely and thoroughly. It was found that these critical concentrations are 1 to 2 M for the acidity. 0.02-0.01 M but preferably about 0.01 M, for the silver cation, and about 0.2 M for the persulfate anion. These findings will be demonstrated below in Example I.

The process of this invention thus comprises providing an aqueous feed solution of radioactive cerium and other lanthanide rare earth isotopes; adjusting the pH value of the feed solution to between 1 and 4; contacting the feed solution with an organic solution of a substantially water-immiscible dialkyl phosphoric acid, whereby the cerium and other lanthanide rare earths are coextracted; separating an organic phase from the depleted aqueous feed solution; contacting said organic phase with an aqueous oxidizing solution 1 to 2 M in mineral acid, 0.02 to 0.01 M in silver ion and about 0.2 M in persulfate anion, whereby cerium is oxidized to the tetravalent state and retained in said organic phase, while said other, tri valent, rare earths are back-extracted into the aqueous solution; separating the aqueous solution from the organic phase; allowing the tetravalent cerium in the organic phase to reconvert to trivalent cerium; contacting the organic phase with a dilute mineral acid stripping solution, whereby the cerium is stripped from the organic phase; and separating said stripped organic phase from the aqueous cerium solution.

Various water-immiscible dialkyl phosphoric acids are suitable for the process of this invention, as is known to those skilled in the art. For instance, di(Z-ethylhexyl) phosphoric acid, bis(2-ethylhexyl) phosphoric acid, and octylphenyl phosphoric acid can be used. The solvents can be employed in undiluted form, or they can be diluted with an organic solvent, such as kerosene, benzene, toluene or xylene. The very best results were obtained with a kerosene solution 0.4 M in di(2-ethylhexyl) phosphoric acid and 0.2 M is tri-n-butyl phosphate.

Before contacting the feed solution with the organic solution for coextraction of the cerium and other rare earths, it is desirable, as has been briefly mentioned, to adjust the pH value to between 1 and 4 for best results. All mineral acids can be used for this purpose, but nitric acid is preferred. The organic phase containing all the rare earth values including the cerium is then separated from the depleted aqueous feed solution. Thereafter the organic phase is contacted with an aqueous solution containing the oxidizing agent in the concentrations of this invention. As has been set forth above, these concentrations are critical and have to be adhered to for optimum results. The persulfate can be used in the form of the acid or as an alkali salt, ammonium, sodium or potassium persulfate being equally suitable.

The silver ion can be added in the form of any water soluble salt, but in the case of a nitric acid solution the nitrate is preferred. As mentioned before, its concentration should range between 0.01 and 0.02 M, the lower concentration being preferred. The oxidizing solution for the back-extraction of rare earths other than cerium has to contain free mineral acid, preferably in a concentration of between 1 and 2 M in the case of nitric acid. While this concentration yields the highest degree of separation, it is not as critical as the concentrations of the persulfate and silver ions; satisfactory results can be obtained with an acidity ranging between 0.5 M and about M. With these conditions and concentrations, an almost quantitative extraction of the trivalent rare earths and an almost quantitative retention of the cerium in the organic phase are accomplished.

Various temperatures have been examined for the separation step just described. While temperatures up to 50 C. yielded fairly satisfactory results and left the tetravalent cerium in this oxidation state for a reasonable period of time, the lower temperatures of 35 and 25 C. were found superior, since they allowed a longer extraction period without self-reduction of cerium. Room temperature was by far the best and is therefore preferably used for this process.

After separation of the cerium-containing organic phase from the aqueous solution containing the trivalent rare earths, the organic phase is allowed to stand for selfreduction of the tetravalent cerium to the trivalent state. This Was found to take approximately hours at room temperature. If it is desired to expedite this reduction, known reducing agents, such as hydrogen peroxide, can be added.

After the reduction is complete, the cerium is stripped from the organic phase with a dilute mineral acid, preferably with nitric acid of a concentration of about 2 M.

All extraction steps described in this specification can be carried out by known means, either in a batch procedure or in a continuous countercurrent operation.

If the trivalent rare earths are to be used, for instance, for medical treatment, it might be desirable to separate them from the sulfate and silver associated with it. This can be done by first adjusting the pH value and then extracting the rare earths from the aqueous solution by means of an organic solvent, for instance di(2-ethylhexyl) phosphoric acid, from which it can then be back-extracted again with a dilute mineral acid. This phase of the process is not part of this invention.

In the following, five examples are given to illustrate the process of this invention.

Example I An aqueous waste solution was used that contained 800 curies per liter of Ce 100 curies per liter of Pm also small amounts of other lanthanide rare earth fission products and Sr, Ru and Zr-Nb The nitric acid concentration was adjusted to a pH value of 1.5 to 1.8.

A series of coextractions were carried out, each time contacting one liter of the aqueous waste solution with 2.2 liters of a kerosene diluent 0.2 M in tributyl phosphate and 0.4 M in di(2-ethylhexyl) phosphoric acid at 35 C. The organic phases obtained in each of these coextraction experiments were used for the studies to be described below for the determination of the operative conditions.

These experiments comprised the extraction of the noncerium trivalent rare earths with an aqueous oxidizing solution containing silver ion, persulfate ion and nitric acid in varying concentrations. The contact temperature was 35 C. and the volume ratio of organiczaqueous stripping solution was 1. After extraction and phase separation, both phases were sampled and analyzed to determine the distribution coefiicients of cerium obtained. In all experiments a persulfate concentration of 0.2 M was used. The results are summarized in Table I.

TABLE I Time of Contaet Percent Ce in Organic Phase 000 FHH 00000 C000 COO PHD- 333 833388 83835 3333 Stowe. oco l-H-H-H-Y Minnow worm to {00 alwipla UCs Dwwfiow woo: A O O HOOQO gag gaggle? 5 39 5 3 3 9 0 2-2 0 z-vs 5 5 000 H 2 0 New 0M NM w r m m a-a-r-r r era-2* errwer O00 O00 000000 @0900 0000 0000 C00 00000 0000 GOO C00 000 000 0 000 0. 0. 0000 0. 0. 00. 0. 000 0. 0. 00 0.

The above data show that, with nitric acid concentrations of 1 M and 2 M, the cerium was practically completely retained in its tetravalent state and thus in the organic phase for the first hour, while thereafter a reduction to the trivalent state occurred within a more or less short period of time which, of course, affected the separation, trivalent cerium being stripped from the organic phase together with the other trivalent lanthanides. However, where the silver ion concentration was 0.01 M, quantitative cerium extraction could be obtained even for five hours with l M nitric acid and an almost quantitative extraction for five hours with 2 M nitric acid. With a nitric acid concentration of 4 M, the cerium was reduced to a remarkable degree after two hours, when the silver concentration was 0.02 M; however, with the optium silver concentration of 0.01 M, a 4 M nitric acid yielded satisfactory cerium retention for five hours. Without the silver catalyst and a nitric acid concentration of 2 M, hardly any cerium oxidation took place, as will be seen from the low cerium extraction into the organic phase.

In some instances the organic phases were furthermore treated for the back-extraction of the cerium into an aqueous cerium solution. For this purpose the organic phases were contacted with nitric acid of a concentration of about 2 M at room temperature. It took usually between and 16 hours to transfer about 98% of the cerium into the nitric acid when no reducing agent was used.

Example II Another series of experiments were carried out similarly to those of Example I but using equal volumes of organic extractant and aqueous feed solutions at 35 C. The organic extractant had the same composition as in Example I, the aqueous strip solution was always 0.2 M in K S O l M in nitric acid, but had varying silver ion concentrations. The results are summarized in Table II.

TABLE II Time of Contact Ag+, M Percent Ce in (min.) Organic Phase These runs show, like those of Example I, that a silver ion concentration of 0.01 M gives by far the best results. Similar results were also obtained with analogous runs in which the nitric acid concentration of the aqueous stripping solutions was 2 M.

Example III In this example the results of several runs made with a stripping solution 0.1 M in potassium persulfate, 0.02 M in Ag+ and 1 M in nitric acid are juxtaposed to the analogous values taken from Table II; the conditions, including composition of the organic extractant, were identical in both cases with the exception that in Table II the persulfate concentration was 0.2 M.

TABLE III Percent Ce in Time of Contact Organic Phase min.)

Kgsgog, M

It is obvious from the above that a persul fate concentration of 0.1 M is not satisfactory for a period of time longer than two hours and that even when the concentration of 0.2 M for the persulfate is superior.

Example IV A number of runs were carried out on non-radioactive feed to study the effect of temperature variations. Temperatures of 25 C., 36 C., and 50 C. were used.

The organic solution was a kerosene solution, 0.4 M in di(Z-ethylhexyl) phosphoric acid, 0.2 M in tributyl phosphate, 0.01 M in cerium and 0.01 M in neodymium. The aqueous stripping solution was 0.1 M in potassium persulfate, 0.02 M in silver nitrate and 1 M in nitric acid. Equal volumes of organic and aqueous solutions were contacted. The results are summarized in Table IV.

TABLE IV Percent Ce in Organic Phase Time (min.)

25 C. 36 C. 50 C.

wiommmh 999999 UIUIQ'POQ 9. 99 OIQrOuAOOU KOlDlDoUI 3. 09 Hofl olbpw The above results demonstrate that some time is necessary to oxidize cerium to the tetravalent organic-extractable state and this time is shorter at elevated temperatures (30 minutes at 25 (3., about five minutes at 36 and 50 C.). The results also show that at 50 C. the cerium retention in the organic solution decreases after approximately two hours, while at 25 C. and 36 C. the extraction still is close to quantitative (99.5%).

Example V An aqueous feed solution containing 35 kilocuries of cerium and 60 kilocuries of Pm and having a pH value of 2 was extracted with kerosene 0.4 M in di(Z-ethylhexyl) phosphoric acid and 0.2 M in tributyl phosphate. The organic phase thus obtained was contacted at room temperature with an aqueous solution 2 M in nitric acid, 0.2 M in potassium persulfate and 0.02 M in silver nitrate. The phases obtained thereby were separated and the aqueous phase was analyzed. It contained only 0.6% of the cerium and all of the promethium.

In the above examples we have employed solvent consisting of di(Z-ethylhexyl) phosphoric acid, tributyl phosphate, and kerosene, which is 0.4 M in di(Z-ethylhexyl) phosphoric acid and 0.2 M in tributyl phosphate.

The di(Z-ethylhexyl) phosphoric acid is the active extractant for the rare earths. While it is substantially insoluble in water, its use in kerosene solution improves the phase separation. The tributyl phosphate acts as a cosolvent for the kerosene and the sodium salt of the di(2-ethylhexyl) phosphoric acid. The latter compound is not present in the solvent during the extraction steps but is formed when the solvent is cleaned up for reuse by treatment with aqueous sodium carbonate. The tributyl phosphate prevents third phase formation.

The di(2-ethylhexyl) phosphoric acid concentration in the mixed solvent may vary from 0.01 M to 1.0 M. The concentration of the tributyl phosphate should be about half that of the di(2-ethylhexyl) phosphoric acid.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process of recovering cerium from radioactive mixtures containing it together with other lanthanide rare earths, comprising providing an aqueous feed solution having a pH value between 1 and 4 from said mixture; contacting the feed solution with an organic solution of a substantially water-immiscible dialkyl phosphoric acid whereby the cerium and other lanthanide rare earths are coeXtracted; separating an organic phase from the depleted aqueous feed solution; contacting said organic phase with an aqueous oxidizing solution 1 to 2 M in mineral acid, 0.02 to 0.01 M in silver ion and about 0.2 M in persulfate anion, whereby cerium is oxidized to the tetravalent state and retained in said organic phase, while said other, trivalent, rare earths are back-extracted into the aqueous solution; separating the aqueous solution from the organic phase; allowing the tetravalent cerium in the organic phase to reconvert to trivalent cerium; contacting the organic phase with a dilute mineral acid stripping solution, whereby the cerium is stripped from the organic phase; and separating said stripped organic phase from the aqueous cerium solution.

2. The process of claim 1 wherein the extractions are carried out at room temperature.

3. The process of claim 2 wherein the solvent solution is a kerosene solution of di(2-ethylhexyl) phosphoric acid and tributyl phosphate.

4. The process of claim 3 wherein the solvent solution is a kerosene solution 0.4 M in di(2-ethylhexyl) phosphoric acid and 0.2 M in tributyl phosphate.

5. The process of claim 1 wherein the aqueous oxidizing solution is a solution 1 M in nitric acid, 0.01 M in silver nitrate and 0.2 M in alkali persulfate.

References Cited UNITED STATES PATENTS 8/1951 Warf 2315 X OTHER REFERENCES MILTON WEISSMAN, Primary Examiner.

OSCAR R. VERTIZ, Examiner.

H. T. CARTER, Assistant Examiner. 

1. A PROCESS OF RECOVERING CERIUM FROM RADIOACTIVE MIXTURES CONTAINING IT TOGETHER WITH OTHER LANTHANIDE RARE EARTHS, COMPRISING PROVIDING AN AQUEOUS FEED SOLUTION HAVING A PH VALUE BETWEEN 1 AND 4 FROM SAID MIXTURE; CONTACTING THE FEED SOLUTION WITH AN ORGANIC SOLUTION OF A SUBSTANTIALLY WATER-IMMISCIBLE DIALKYL PHOSPHORIC ACID WHEREBY THE CERIUM AND OTHER LANTHANIDE RARE EARTHS ARE COEXTRACTED; SEPARATING AN ORGANIC PHASE FROM THE DEPLETED AQUEOUS FEED SOLUTION; CONTACTING SAID ORGANIC PHASE WITH AN AQUEOUS OXIDIZING SOLUTION 1 TO 2 M IN MINERAL ACID, 0.02 TO 0.01 M IN SILVER ION AND ABOUT 0.2 M IN PERSULFATE ANION, WHEREBY CERIUM IS OXIDIZED TO THE TETRAVALENT STATE AND RETAINED IN SAID ORGANIC PHASE, WHILE SAID OTHER, TRIVALENT, RARE EARTHS ARE BACK-EXTRACTED INTO THE AQUEOUS SOLUTION; SEPARATING THE AQUEOUS SOLUTION FROM THE ORGANIC PHASE; ALLOWING THE TETRAVALENT CERIUM IN THE ORGANIC PHASE TO RECONVERT TO TRIVALENT CERIUM; CONTACTING THE ORGANIC PHASE WITH A DILUTE MINERAL ACID STRIPPING SOLUTION, WHEREBY THE CERIUM IS STRIPPED FROM THE ORGANIC PHASE; AND SEPARATING SAID STRIPPED ORGANIC PHASE FROM THE AQUEOUS CERIUM SOLUTION. 